1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device for improving moving picture quality and a method for driving the same.
2. Description of the Related Art
Since a liquid crystal display (hereinafter referred to as an LCD) is lightweight and slim and has low power consumption, its fields of application are broad. Accordingly, the LCD is used in an office automation device, an audio/video device, etc. On the other hand, the LCD displays a desirable image on a screen by adjusting an amount of light beam transmission according to image signals that are applied to a plurality of switches disposed in a matrix. Since the viewers pursue better moving picture quality, a liquid crystal material or a driving method has been under development attempting to meet the viewers' requirements.
A cathode-ray tube (CRT) utilizes an impulsive type light source that emits light by an injection of an electron gun. The LCD utilizes a hold type light source that emits light by a backlight system that employs a linear lamp (a fluorescent lamp) as a light source. Therefore, it is difficult for the LCD to display perfect moving pictures. That is, when moving pictures are displayed on the LCD, its hold property causes deterioration in an outline of an image. Thus, the image quality deteriorates (e.g., motion blurring occurrence). To prevent the moving picture outline deterioration, there is provided an LCD according to a backlight scanning method that employs a direct-type backlight including a plurality of lamps disposed across the LCD.
FIG. 1 is a block diagram of a driving device in a related art LCD device. FIG. 2 is a sectional view of a backlight unit and an LCD panel of FIG. 1.
Referring to FIGS. 1 and 2, the driving device in an LCD device according to a related art backlight scanning method includes an LCD panel 2, a data driving unit 4, a gate driving unit 6, a backlight unit 10, a lamp driving unit 12, and a timing controller 8. The LCD panel 2 includes data lines and gate lines, which are intersected and the TFTs are formed adjacent to the intersection points. The data driving unit 4 supplies data to the data lines of the LCD panel 2. The gate driving unit 6 supplies gate pulses to the gate lines of the LCD panel 2. The backlight unit 10 projects light on the LCD panel 2 by sequentially driving a plurality of lamps 30. The lamp driving unit 12 controls the backlight unit 10. Additionally, the timing controller 8 controls the data driving unit 4 and the gate driving unit 6 and simultaneously drives the lamp driving unit 12.
As illustrated in FIG. 2, the backlight unit 10 includes a plurality of lamps 30, a lamp housing 22 surrounding the plurality of lamps 30, and a diffusion plate 20 covering the lamp housing 22.
The plurality of lamps 30 are sequentially driven in response to a control of the lamp driving unit 12. The lamp housing 22 surrounds the plurality of lamps 30, and also reflects the light emitted from the plurality of lamps toward the diffusion plate 20 by using a reflective surface 24. The LCD panel 2 includes two glass substrates and liquid crystal interposed therebetween. The TFT is formed adjacent to an intersection of the data line and the gate line in the LCD panel 2, and supplies the data into a liquid crystal cell through the data line in response to a scanning pulse outputted from the gate driving unit 6. A source electrode of the TFT is connected to the data line, and a drain electrode of the TFT is connected to a pixel electrode of the liquid crystal cell. Additionally, a gate electrode of the TFT is connected to the gate line. The LCD panel 2 is stacked on the diffusion plate 20 of the backlight unit 10.
The timing controller 8 rearranges the digital video data supplied from a digital video card (not shown) by red R, green G, and blue B. The data RGB rearranged by the timing controller 8 is supplied to the data driving unit 4. Additionally, the timing controller 8 generates a data control signal and a gate control signal by using horizontal/vertical synchronization signals H and V inputted into the timing controller 8. The data control signal includes a dot clock Dclk, a source shift clock SSC, a source enable signal SOE, a polarity signal POL, etc., and also supplies them to the data driving unit 4. The gate control signal includes a gate start pulse GSP, a gate shift clock GSC, a gate output enable GOE, etc., and also supplies them to the gate driving unit 6. Additionally, the timing controller 8 controls the lamp driving unit 12 to sequentially drive the backlight unit 10 when data is completely supplied to a liquid crystal cell.
After sampling the data in response to a data control signal from the timing controller 8, the data driving unit 4 latches the sampled data by one line, and then converts the latched data into an analog gamma voltage from a gamma voltage supplying unit (not shown).
The gate driving unit 6 includes a shift register and a level shift. The shift register sequentially generates gate pulses in response to a gate start pulse GSP in gate control signals outputted from the timing controller 8. The level shift shifts a voltage level of the gate pulse into a voltage level appropriate for driving a liquid crystal cell.
The lamp driving unit 12 sequentially drives the plurality of lamps 30 of the backlight unit 10 in response to a lamp driving control signal from the timing controller 8. That is, the lamp driving unit 12 completely supplies a data voltage into the liquid crystal cell, and then sequentially drives the plurality of lamps 30.
FIG. 3 illustrates a method for driving scanning in a related art backlight unit. FIGS. 4A and 4B illustrate a ghost phenomenon in the first and eighth lamps of FIG. 3.
Referring to FIGS. 3, 4A and 4B, when a vertical synchronization signal Vsync dividing each image frame is applied to an LCD panel, the gate output signals are sequentially applied to the gate lines corresponding to one frame in the LCD panel.
At this point, the plurality of lamps in the backlight unit are sequentially driven in synchronization with a drive signal applied to the gate line. That is, the first lamp changes from an on-state into an off-state when a gate output signal is applied to one of the N number of gate lines to supply a data voltage into a pixel region of the LCD panel. Additionally, the second lamp changes from an on-state into an off-state when a gate output signal is applied to another gate line, which is next to the gate line corresponding to the first lamp, to supply the data voltage into a pixel region of the LCD panel completely.
Accordingly, as illustrated in FIG. 3, when the gate output signal is applied to the gate line, the first to eighth lamps are sequentially turned on/off with the same interval t1.
However, the direct-type LCD device including a plurality of lamps has respectively different temperatures in different regions due to a convection phenomenon in the backlight unit.
A temperature difference between the first gate line region and the Nth gate line region is 10° C. or higher. The temperature difference causes the difference of the liquid crystal response time . Therefore, a pixel response time of the Nth gate line region is slower than that of the first gate line region.
The pixel response time difference due to the temperature difference causes a ghost phenomenon because an interval of a lamp in an on-state is broad even if a pixel region is opened.
The first lamp region corresponding to the first gate line region has a higher temperature than the eighth lamp region corresponding to the last gate line region. Thus, the pixel response time of the first lamp region is faster than that of the eighth lamp. As illustrated in FIG. 4A, when the first lamp is turned on or off, the on/off property (response time) of the pixel region in the LCD panel is in a pixel-on-state until the first lamp is turned on for the next image frame. When a pixel is in an off-state, the first lamp of the next image frame is turned on. At this point, when the pixel region is in an off-state, there is an overlapping interval where the lamp is in an on-state. Therefore, the ghost phenomenon occurs.
However, as illustrated in FIG. 4B, in a region having relatively lower temperature than the first lamp region, a pixel region is in an on-state until the eighth lamp is turned on. Therefore, since the liquid crystal response time becomes slower due to the low temperature, it takes more time to change from an on-state into an off-state. Consequently, a ghost region becomes broader than before.
When the ghost region becomes broader, an afterimage occurs when displaying a moving picture, thereby deteriorating the image quality. In particular, since the eighth lamp region in a low temperature is a region displaying the subtitle in an LCD TV, it is difficult to identify the subtitle due to the afterimage.